1. Field of the Invention
The invention relates to a technique for measuring depth geometries on a semiconductor substrate, and particularly to determining depth geometries of recesses and dielectric layers relative to a reference interface at high resolution by detecting reflected rays of an incident broadband light source and determining the depth geometries based on an interferometric analysis of a measured dependence of reflectance intensity on wavelength.
2. Discussion of the Related Art
Many techniques have been developed for measuring thin film thicknesses and depths of structures on semiconductor substrates. Among the most widely practiced of them is Fourier Transform Infrared Spectroscopy (FTIR). See, e.g., Takada, et al., Trench Depth Measurement System for VLSI DRAM's Capacitor Cells Using Optical Fiber and Michelson Interferometer, Journal of Lightwave Technology, Vol. LT-5, No. 7, 881 (July 1987). Transmission and reflection spectroscopy may be performed using this technique.
The trench depth measurements performed by Takada et al. are on substrates having trenches wherein reflections from only two interfaces, the trench bottoms and the non-recessed silicon substrate, are detected and a wavelength spacing between peaks of an interferogram is measured to determine the depth of the trenches relative to the silicon substrate surface. The technique is not a satisfactory method of measuring trench depth, however, when a dielectric layer is either in the trench or over the silicon substrate. A measurement of the wavelength spacing between peaks in this case yields an indeterminate trench depth value because a reflection component from the dielectric surface changes the measured peak spacing in the interferogram. This peak spacing change, in turn, causes the trench depth to be measured relative to a "virtual" interface lying at an unknown depth different from either of the silicon substrate or dielectric layer surfaces.
Moreover, when the dielectric layer thickness is changing, the wavelength spacing between the peaks of the interferogram will change as the dielectric layer thickness changes, yielding a changing trench depth value, even when the trench depth is not actually changing. When the trench depth is changing along with the thickness of the dielectric layer, the peak spacing change will yield a trench depth change rate which differs from that which is actually occurring. Since it is important to know trench depths with respect to interfaces of known depth having one or more dielectric layers of unknown thickness above them, a more satisfactory technique is needed.
Additionally with respect to conventional FTIR techniques, they do not have sufficient resolution to accurately measure shallow trench depths. In fact, a minimum detectable trench depth using the technique of Takada et al., e.g., is approximately 1.5 microns with an error of +/-0.2 microns. This is because, as mentioned, Takada et al. merely calculates a frequency spacing between adjacent peaks in their FTIR spectra to determine trench depths. Furthermore, FTIR techniques such as those performed by Takada et al. cannot achieve better resolution without using a far broader band incident radiation source. Finally, FTIR techniques such as those performed by Takada et al. do not work well when additional layers are formed on the silicon substrate because overlapping frequencies erode the resolution of the technique further.
Another technique for measuring layer thicknesses and trench depth uses a monochromatic light source to irradiate a trench bottom or a layer on a substrate, and plots the reflected intensity versus time. See, e.g., W. G. Breiland and K. P. Killeen, A Virtual Interface Method for Extracting Growth Rates and High Temperature Optical Constants from Thin Semiconductor Films Using In Situ Normal Incidence Reflectance, J. App. Phys. 78 (11), pp. 6726-36 (December 1995). As the layer thickness or trench depth changes, the plot passes through a periodic series of maxima and minima whose temporal positions depend on the thickness or trench depth at any given time. When measuring trench depths, however, this technique does not take into consideration a changing thickness of a dielectric layer above a fixed interface, such as, e.g., a photoresist layer above a silicon substrate or an oxide layer in a recess. Moreover, the measurement cannot be performed instantaneously.
A technique for performing microdensitometric measurements of linewidths and spectral interferometric measurements of thicknesses is disclosed in U.S. Pat. No. 4,674,883 to Baurschmidt. The technique is designed particularly for measuring line widths, rather than depths. Baurschmidt discloses to scan the surface of a wafer using a collecting lens to gather scattered light. The collecting lens gathers light reflected at a wide range of reflection angles. Gathered light is focused to a slit, spectrally dispersed and detected by a position sensitive array detector. The technique exhibits a resolution of around 10 microns due to the large numerical aperture of the lens and wide range of detected reflection angles of detected reflected rays. The resolution of this technique is not satisfactory for performing depth measurements of structures such as shallow trenches and recesses.
Profilometry techniques have been used to measure thicknesses of thin films. A stylus in contact with a surface is drawn over a step at the edge of a thin film and the step height measured. Depths of wide structures such as wide trenches and/or recesses may also be measured, but limitations on the size of the stylus used to perform the technique restrict applicability of the technique, so that depths of narrow structures such as narrow trenches are not measurable. Moreover, profilometry is a mechanical, rather than an optical technique, and contact by the stylus disturbs structures and films on a silicon substrate when measurements are performed.
Today shallow trench isolation (STI) is the preferred method for isolating MOS-transistors and other devices on silicon. STI allows smaller structure processing than its predecessor, localized oxidation of silicon (LOCOS), which relied upon a complex process including initially growing silicon dioxide on a silicon substrate. An illustration of the STI technique is shown in FIG. 1. When the trenches 2 are filly etched (not as shown), they will reach thousands of angstroms into the silicon substrate. The trenches 2 will serve to isolate transistor devices on a completed chip, because the trenches 2 will be filled with insulating material. Most typically, a material such as oxide is deposited into the trench 2 to fill the trench 2 and electrically isolate devices on either side of it.
Before etching of the trench structures 2 begins, a layer of photoresist 4 is formed. A stepper next photolithographically exposes portions of the photoresist to ultraviolet radiation in a predetermined pattern. A developer is next poured on to remove either the exposed or non-exposed portions of the photoresist. A dry etch next removes material not protected by photoresist, i.e., under places where the photoresist was removed in the developing step. In principle, this etching step etches, e.g., the trenches, above which no photoresist layer remains, while leaving unscathed the photoresist layer and material beneath it. However, during a real dry etching process, the photoresist layer is also stripped away, albeit at a slower rate than the depth of the trenches is increased. This is why a satisfactory trench depth measurement technique will ultimately measure a trench depth with respect to a fixed interface beneath the photoresist layer, rather than with respect to the air/photoresist interface 7.
Trench depths may be first measured with respect to the air/photoresist interface 7. Thereafter, a Thickness of the photoresist layer may be subtracted, and the result taken to be the distance from the bottom of the trench to the bottom of the photoresist layer. To do this accurately, a precise thickness of the photoresist layer 4 must be determined. Since the photoresist layer thickness changes during the etch, unless the photoresist layer thickness can be measured at the time of the trench depth measurement, the accuracy of any trench depth determination will be unsatisfactory. For shallow trenches, the accuracy of the photoresist layer thickness measurement is even more important than it is for deeper structures.
Poly recess on oxide processing is another prevalent technique wherein accurate recess depth determinations are desired. Recess structures are first etched into an oxide layer. Next, a layer of poly is deposited onto the oxide to both fill the recesses and to form a planar layer of poly over the oxide. A subsequent etch then first strips away the planar poly. Then, both the oxide and the poly filling the recesses is etched. At this time, the poly and the oxide are etched at different rates. It is desired to have a technique for measuring the recess depth and the oxide layer thickness, each with respect to a fixed interface such as the silicon substrate or the poly.